Power amplifying device comprising a stabilizing circuit

ABSTRACT

A power amplifying device includes a stabilizing circuit between an input terminal and an amplifier. The stabilizing circuit has a first line, a second line, and a third line. The first line is connected to the ground. The length of the first line is equal to or longer than three fourths of the wavelength of the operating frequency.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to power amplifying device, and inparticular, to power amplifying device for millimeter and micrometerwave bands.

2. Background Art

In Japanese Unexamined Patent Publication No. 2002-353745, a radiofrequency power amplifying device for millimeter and micrometer wavebands is described, which comprises a stabilizing circuit disposed nearthe transistor in order to allow stable operation.

When the stabilization circuit is disposed in the power amplifyingdevice, there is a possibility not to be able to stabilize enough if thechange of the characteristic of the transistor is not considered. Thatis, to attain satisfactory stabilization, it is necessary to take intoconsideration the changes of characteristics of the transistor. However,there exists a trade-off relation between the stability and theamplifying capability. Particularly, in the millimeter wave band, if nocharacteristics margin is given to the transistor, designing thestabilizing circuit so as to secure stability regardless of changes ofthe characteristics of the transistor is likely to make useless much ofits potential as an amplifying device.

SUMMARY OF THE INVENTION

The present invention has been developed to solve the above-describedproblems, and therefore it is an object of the present invention toprovide a power amplifying device capable of exhibiting stable outputperformance without sacrificing much of its potential as an amplifyingdevice.

The above object is achieved by a power amplifying device comprising aninput terminal and an output terminal, an amplifying device disposedbetween the input terminal and the output terminal, and a stabilizingcircuit disposed between the input terminal and the amplifier, whereinthe stabilizing circuit comprises the first line which is equal to orlonger than three fourths of the wavelength of the operating frequency,and the first line is connected to the ground.

According to the present invention, it is possible to attain a poweramplifying device capable of providing stable output performance withoutsacrificing much of its potential as an amplifying device.

Other features and advantages of the invention will be apparent from thefollowing description taken in connection with the accompanyingdrawings.

BRIEF DESCRIPTION OF DRAWINGS

FIGS. 1 and 2 show the circuit configurations of a power amplifyingdevice of the first embodiment;

FIG. 3 shows the frequency characteristics of the MAG (Maximum AvailableGain) and MSG (Maximum Stable Gain) of the transistor;

FIG. 4 shows the attenuation characteristic of the stabilizing circuitof the first embodiment;

FIG. 5 shows a small signal characteristic of the 77 GHz band amplifyingdevice not provided with the stabilizing circuit;

FIG. 6 shows the small signal characteristics of the 77 GHz bandamplifying device provided with the stabilizing circuit of the firstembodiment;

FIG. 7 shows a circuit configuration of a power amplifying device of thesecond embodiment;

FIG. 8 shows the attenuation characteristic of the stabilizing circuitof the second embodiment; and

FIG. 9 shows a circuit configuration of a power amplifying device formedby using a FET of the second embodiment.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Embodiments of the present invention will be described below referringto the drawings. In the drawings, the same or equivalent parts will bedenoted by the same reference numerals, and the description thereof willbe simplified or omitted.

First Embodiment

FIG. 1 shows the circuit configuration of a power amplifying device inaccordance with this embodiment of the present invention. An amplifier 2is disposed between an input terminal (IN) and an output terminal (OUT).Further, a stabilizing circuit 1 is disposed between the input terminal(IN) and the amplifier 2. The stabilizing circuit 1 has lines L₁ (thefirst line), L₂ (the second line) and L₃ (the third line) whose lengthsare l₁, l₂ and l₃, respectively. From the input terminal (IN) to theoutput terminal (OUT), the lines L₂ and L₃ are connected in series. Theline L₃ is connected to the amplifier 2. One end of the line L₁ isconnected to between the lines L₂ and L₃. The other end of the line L₁is connected to a ground (GND), that is, earthed. The length l₁ of theline L₁ is equal to or longer than three fourths of the wavelength λ ofthe operating frequency. The characteristic impedance of each of thelines L₂ and L₃ is basically made equal to the characteristic impedance(usually 50 ohm) of the system although it may have a different valuesince these lines are used to adjust characteristics of the amplifier 2.

FIG. 2 shows the configuration of the amplifier 2 shown in FIG. 1. (Inthis figure, the stabilizing circuit 1 of FIG. 1 is omitted.) From aninput terminal (IN) to an output terminal (OUT), a matching circuit 2 a,a stabilizing circuit 2 b, a field effect transistor (hereinafter,denoted as the “FET”) 2 c and a matching circuit 2 d are connectedserially.

FIG. 3 shows the frequency characteristics of the MAG (Maximum AvailableGain) and MSG (Maximum Stable Gain) of the transistor. Assume that theamplifier is designed to have its operating frequency band near theboundary between the MAG region (stable region) and MSG region (unstableregion) of the transistor. In MGA region A, since the transistor itselfis stable, it is not necessary to stabilize the output performance ofthe amplifier. In MSG region B, however, since the transistor itself isunstable, it is necessary to stabilize the output performance of theamplifier. Therefore, the amplifier must be designed so as to maintainthe stability of its output performance even if characteristics of thetransistor vary to shift the boundary between MSG region B and MAGregion A.

Generally, however, stability of the output performance of an amplifierhas a trade-off relation with its amplifying capability. Particularly inthe millimeter wave band, if the transistor has no characteristicsmargin, giving excessive importance to the stability of the outputperformance is likely to make it impossible to attain sufficientamplifying capability. Reversely, giving priority to the amplifyingcapability may result in unstable output performance of the amplifierdepending on the conditions. Therefore, in the present embodiment, thestabilizing circuit 1 (see FIG. 1) is provided between the inputterminal (IN) and output terminal (OUT) of the amplifier 2 whilestabilization by the stabilizing circuit 2 b near the transistor 2 isminimized.

FIG. 4 shows the attenuation characteristic of the stabilizing circuit 1itself. The operating frequency of the amplifier is 77 GHz. The lengthl₁ of the line L₁ is given by l₁=(7/4)·λ, wherein λ is the wavelength ofthe operating frequency. If such a millimeter wave band frequency isused, it is highly probable that an unstable region would be left belowand near the operating frequency band. Therefore, l₁ must be longer thana half of the wavelength λ. In practical application, it is necessary toset the length equal to or longer than three fourths of the wavelengthλ.

As shown in FIG. 4, the stabilizing circuit 1 functions as a periodicbandpass filter exhibiting cutoffs in a DC to low frequency region. Ithas a cutoff near 22 GHz which is a frequency at which l₁ is a half ofthe wavelength λ. Further, cutoffs appear periodically at multiples of22 GHz (44 GHz, 66 GHz, 88 GHz, . . . ). There is no attenuation at eachmiddle point (11 GHz, 33 GHz, 55 GHz, 77 GHz, . . . ) between cutofffrequencies.

Accordingly, the stabilizing circuit 1 shows the characteristic ofpassing around the operating band (76 GHz-77 GHz) without attenuation.That is, the stabilizing circuit 1 does not deteriorate the gain in theoperating band. In addition, it has almost no influence on the matchingand reflection characteristics in the operating band. It is thereforepossible to stabilize the output performance of the amplifier withoutlowering the amplifying ability around the operating frequency. Notethat since this circuit is a passive circuit whose characteristics aresubstantially determined by the length l₁ of L₁, manufacturing can bedone very steadily once the characteristics are grasped throughexperimental fabrication.

Then, small signal characteristics of the amplifier not provided withthe stabilizing circuit 1 were compared with those of the amplifierprovided with the stabilizing circuit 1. FIG. 5 shows small signalcharacteristics of the 77 GHz band amplifier not provided with thestabilizing circuit 1. Without the stabilizing circuit 1, the K factorfalls between 67 and 70 GHz below and near the operating band (77-78GHz).

FIG. 6 shows small signal characteristics of the 77 GHz band amplifierprovided with the stabilizing circuit 1. As compared with those (seeFIG. 5) of the above one not provided with the stabilizing circuit 1,the fall of the K factor is suppressed in the 65-75 GHz region. Asunderstood from this result, it is possible to stabilize the outputperformance of the amplifier by adding the stabilizing circuit 1 toimprove the K factor below and near the operating band.

Although in the present embodiment, cutoff attenuation around 66 GHz isutilized to stabilize the amplifier at frequencies below and near theoperating band, the output performance of the amplifier can bestabilized at around other cutoff frequencies (11 GHz, 33 GHz, 55 GHzand so on), too.

Second Embodiment

FIG. 7 shows the circuit configuration of a power amplifying device inaccordance with a second embodiment of the present invention. Thefollowing description is focused on what is different from the firstembodiment. As shown in FIG. 7, a capacitor C₁ is provided between theline L₁ of the stabilizing circuit 1 and the ground (GND). FIG. 8 showsattenuation characteristics of the stabilizing circuit 1 plotted byshifting the capacitance of the capacitor C₁ in a given range.

As shown in FIG. 8, as the capacitance of the capacitor C₁ is shifted,the fully passed frequencies and cutoff frequencies shift according tothe capacitance. It is therefore possible to stabilize the outputperformance of the amplifier at desired frequencies by appropriatelysetting the capacitance of the capacitor C₁.

Shown in FIG. 9 is an example where the above mentioned capacitor C₁ isformed by using a FET. The FET (T₁) is provided between the line L₁ andthe ground (GND). The source or drain terminal of T₁ is connected to theline L₁ and the other terminal is connected to the ground. That is, theoff-capacitance between the source and the drain is used as thecapacitor C₁. Similar to the power amplifying device of FIG. 7, it istherefore possible to stabilize the output performance of the amplifierat desired frequencies by appropriately setting the off-capacitance.

In addition to the effect attained by the first embodiment, the poweramplifying device in accordance with the present embodiment canstabilize the output performance of the amplifier at desiredfrequencies.

Third Embodiment

In the stabilizing circuit 1 of the first embodiment, the length l₁ ofthe line L₁ is fixed to a certain length equal to or longer than threefourths of the wavelength of the operating frequency. In the case of thepresent embodiment, it is possible to choose an appropriate lengthaccording to critical frequencies. That is, the length l₁ of the line L₁can be changed by selection so as to stabilize the output performance ofthe amplifier at desired frequencies.

For instance, in the first embodiment, the line L₁ has a fixed lengthlonger than at least a half of the wavelength λ of the operatingfrequency so that cutoff attenuation around 66 GHz is utilized tostabilize the 77-78 GHz band amplifier at frequencies below and near theoperating band. In the case of the present embodiment, an appropriateline length can be chosen according to the operating frequency band ofthe amplifier regardless of the operating frequency band. If an unstableregion occurs without the use of the stabilizing circuit 1, it ispossible to stabilize the output performance at frequencies in andaround the region by selecting a line length appropriate to the unstableregion.

In addition to the effect attained by the first embodiment, the poweramplifying device in accordance with the present embodiment canstabilize the output performance of the amplifier at desiredfrequencies.

Obviously many modifications and variations of the present invention arepossible in the light of the above teachings. It is therefore to beunderstood that within the scope of the appended claims the inventionmay by practiced otherwise than as specifically described.

The entire disclosure of a Japanese Patent Application No. 2005-338509,filed on Nov. 24, 2005 including specification, claims, drawings andsummary, on which the Convention priority of the present application isbased, are incorporated herein by reference in its entirety.

1. A power amplifying device comprising: an input terminal and an outputterminal; an amplifier disposed between the input terminal and theoutput terminal and amplifying a signal at an operating frequency; and astabilizing circuit disposed between the input terminal and theamplifier, wherein the stabilizing circuit comprises a first line havinga length equal to or longer than three fourths of the wavelength of theoperating frequency, and the first line is connected to the ground. 2.The power amplifying device according to claim 1, wherein thestabilizing circuit comprises a second line and a third line connectedin series between the input terminal and the amplifier, and a first endof the first line is connected between the second line and the thirdline, and a second end of the first line is connected to the ground. 3.The power amplifying device according to claim 2, wherein the firstline, the second line, and the third line compose a T-shaped circuit. 4.The power amplifying device according to claim 2, wherein the secondline and the third line have a predetermined impedance.
 5. The poweramplifying device according to claim 3, wherein the second line and thethird line have a predetermined impedance.
 6. The power amplifyingdevice according to claim 1, including a capacitor provided between thefirst line and the ground.
 7. The power amplifying device according toclaim 2, including a capacitor provided between the first line and theground.
 8. The power amplifying device according to claim 3, including acapacitor provided between the first line and the ground.
 9. The poweramplifying device according to claim 4, including a capacitor providedbetween the first line and the ground.
 10. The power amplifying deviceaccording to claim 5, including a capacitor provided between the firstline and the ground.
 11. The power amplifying device according to claim6, wherein the capacitor is provided by a field effect transistor havinga source and a drain providing the capacitance.
 12. A power amplifyingdevice comprising: an input terminal and an output terminal; anamplifier disposed between the input terminal and the output terminal;and a stabilizing circuit disposed between the input terminal and theamplifier, wherein the stabilizing circuit comprises a line which has alength selectable according to the wavelength of a desired frequency,and the line is connected to the ground.